Why should you care?

ShopNest promo rules like “SKU starts with HD and price > 1000” need a tiny expression tree.

See it live — copy this example

Paste into a C# console or class library project and run dotnet run.

public interface IRule { bool Eval(Item item); }
public sealed record Item(string Sku, decimal Price);
public sealed class SkuPrefixRule : IRule
{
    private readonly string _prefix;
    public SkuPrefixRule(string p) => _prefix = p;
    public bool Eval(Item i) => i.Sku.StartsWith(_prefix);
}
public sealed class MinPriceRule : IRule
{
    private readonly decimal _min;
    public MinPriceRule(decimal m) => _min = m;
    public bool Eval(Item i) => i.Price >= _min;
}
public sealed class AndRule : IRule
{
    private readonly IRule _a, _b;
    public AndRule(IRule a, IRule b) { _a = a; _b = b; }
    public bool Eval(Item i) => _a.Eval(i) && _b.Eval(i);
}

IRule promo = new AndRule(new SkuPrefixRule("HD"), new MinPriceRule(1000));
Console.WriteLine(promo.Eval(new Item("HD-100", 4999)));

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• Leaf rules check one fact; AndRule composes them.
• That is a tiny interpreter over a rule AST.
• For complex grammars prefer a mature library.

Practice next

1. Evaluate the sample item true/false.
2. Add OrRule.
3. Parse a simple string into rules later.
4. Add NotRule.
5. Fail HD-50 at price 500.

Remember

AST + Eval. Compose rules. Keep scope tiny.

ShopNest promo rule tree

Marketing composes prefix + min-price rules.

Outcome: Promos change without redeploying ifs everywhere.